Sheathing is a basic and necessary part of exterior wall construction. Early sheathing usually comprised 8 inch wide board nailed horizontally with shiplapped top & bottom edge to facilitate joinder. The board was usually covered with a layer of tar paper. Approximately 20 years ago, board was replaced by plywood as the most common sheathing material. More recently fiber board has replaced plywood as the most common sheathing material.
Resulting from concern about the cost and availability of energy, thermal insulative quality is an increasingly important consideration in selecting sheathing materials. Frequently used, for example, are urethane foam and expanded polystyrene having insulative value at least four times that of fiber board.
Two problems arise with the use of high insulative sheathing. First, it fails to ridigify the "two-by-four" structure wall framework in the manner typical of prior sheathing. Use of metal or wood corner bracing is a convenient solution. The second problem arises from the fact that high insulative sheathing is nonpermeable to moisture. In colder climates, warm and humid inside air infiltrates the wall, causing moisture condensation on portions of the wall framework and sheathing having temperature below the dew point for the inside air. Due to the barrier formed by the sheathing, moisture is unable to migrate further outward and collects inside the wall, reducing the insulative efficiency of any fibrous insulation between frame members, and possibly damaging the wall.
Sufficient movement of air within the wall evaporates such moisture, thereby preventing its accumulation. Usually the tendency of warm air to rise in the wall provides sufficient air movement. One method of maintaining an air flow is to install corrugated plastic vent strips between the sheathing and each horizontal frame member of the framework. The vent strips maintain spaces between the sheathing and frame members to allow free upward movement of air to the structure attic.
Of course the cost of installing sheathing increases due to the extra material and labor in mounting the vent strips. A more serious hazard is the practice of applying high insulative sheathing without use of vent strips, usually due to failure to recognize the problem of moisture accumulation. Moreover, a contractor may disregard instructions and install sheathing without strips or other vent means, saving money initially, but thereby causing serious long range consequences.